In large mixing bowl, combine 2 cups cake flour, 3/4 cup sugar, 2 teaspoons baking powder, 3/4 teaspoon cinnamon, and 1/2 teaspoon kosher salt. Whisk to combine all ingredients and set aside.
In a 4 cup measuring cup, combine 3/4 cup buttermilk, 2 eggs, 1 teaspoon vanilla extract, and 2 tablespoons melted butter. Whisk to combine.
Pour wet ingredients into dry ingredients and stir until just combined. The batter will be thick. As soon as all ingredients are combined, set the batter aside and walk away. The batter needs to rest about 15 minutes before using to ensure fluffy, donuts with a lighter texture.
While the batter is resting, preheat oven to 425°F and grease donut pan with butter or spray with cooking spray.
When the oven is preheated, fill the cups of the donut pan approximately 2/3 full. Don't stir the batter, just spoon it into the donut cups and smooth the top.
Bake in the preheated oven for 7-9 minutes. Remove and allow to cool in pan for 5 minutes, and then carefully remove the donuts to a cooling rack. I use a spoon or a butter knife to gently get underneath the donut to lift it from the pan.
Continue baking the donuts in this manner until all the batter is used.
To coat the donuts with cinnamon sugar, combine 2 tablespoons sugar with 1 teaspoon cinnamon. Lightly brush each donut with melted butter and dip in the cinnamon sugar mixture.